Presently, data communications devices ("DCDs") and other data communication equipment ("DCE"), such as modems, for example, are used to transport digital data between data terminal equipment such as personal computers and workstations, over channels such as a General Switched Telephone Network ("GSTN"). Typically, the communication signal includes digital data in the form of a carrier signal that is modulated by a predetermined transmit constellation of signal points. A set of signal points taken from the transmit constellation represents a digital code or value to be transported. At the local DCD, a carrier signal is thus modulated with the set of constellation signal points corresponding to the digital code or value to be transmitted over the channel. The channel may introduce amplitude distortion, phase distortion, additive noise, carrier frequency offset, and other distortions or impairments. The received signal, having been distorted by the channel and having, for example, amplitude and phase distortions, should be equalized (or compensated) by the receiver in order to increase the accuracy of data transmission and increase the rate of data transfer over the channel.
Adaptive equalizers are known in the prior art and are used to provide the compensation to reduce the effects of distortion caused by a transmission channel. Digital Signal Processors, such as a Motorola M56002, may be used to implement an equalizer. Detailed descriptions of adaptive equalizers may be found throughout engineering literature, such as S. Qureshi, "Adaptive Equalization", Proceedings of the IEEE, Vol. 73, No. 9, pp. 1349-1387 (September, 1985). Equalizers typically have a predetermined, fixed number of indexed coefficients, referred to as an indexed set of equalizer coefficients, which may be indexed numbers such as a sequence {c(0), c(1), . . . , c(k)}. In order to compensate for channel distortions, an equalizer should be initialized during a training period, i.e., the equalizer should start the compensation process with initial numerical values for the coefficients. While adaptive equalization is known in the art, however, there is little information available concerning establishing or setting these initial values for these equalizer coefficients.
A prior art method for initializing equalizer coefficients is to arbitrarily set one of the coefficients (usually near the middle of the sequence) to a numerical value of one and set all of the remaining coefficients to values of zero. This initialization method tends to be adequate for GSTN connections both having minimum distortions and having sufficient training time to allow an adaptive algorithm (such as an LMS algorithm) to generate more accurate coefficient values. If the distortions are more significant, or a shorter period of time is required or advisable for training, then this more arbitrary initialization method may be insufficient for proper equalization. Another prior art method for initializing equalizer coefficients is to set all of the coefficients to predetermined numerical values corresponding to a channel having typical or average characteristics. This initialization method may be adequate for equalizing channels having typical characteristics, but may not allow the adaptive algorithm to generate coefficients for channels having more atypical impairments, such as either minimum distortions or significant distortions.
As a result, there continues to be a need to provide an apparatus and method for generating equalizer coefficients to more accurately initialize an equalizer during a potentially short period of time for training, in order to subsequently permit an adaptive algorithm to generate coefficients to equalize for a broad spectrum of channel distortions and other impairments.